This invention relates generally to electromagnetic flowmeter systems, and in particular to a system in which the electromagnet is energized by a low-frequency, square-wave power.
In an electromagnetic flowmeter, the liquid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field normal to the direction of flow being established by an electromagnet. When the flowing liquid intersects this field, a signal is induced therein which is transferred to the electrodes. This signal, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator.
The magnetic field may be either direct or alternating, for in either event the amplitude of signal induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the d-c signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Though a-c operation is clearly advantageous in that polarization is obviated and the flow-induced a-c signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications. The two spurious voltages that are most troublesome are:
1. stray capacitance-coupled voltages from the coil of the electromagnet to the electrodes, and PA1 2. induced loop voltages in the input leads. The electrodes and leads in combination with the liquid extending therebetween constitute a loop in which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by electrostatic shielding and by low-frequency excitation of the magnet to cause the impedance of the stray coupling capacitance to be large. But the spurious voltage from the second source is much more difficult to suppress.
The spurious voltage resulting from the flux coupling into the signal leads is usually referred to as the quadrature voltage, for it is assumed to be 90.degree. out of phase with the a-c flow-induced voltage. Actual tests have indicated that this is not true in that a component exists in-phase with the flow-induced voltage. Hence, that portion of the "quadrature voltage" that is in-phase with the flow-induced voltage signal constitutes an undesirable signal that cannot readily be distinguished from the flow-induced signal, thereby producing a changing zero shift effect.
The calibration of existing types of a-c operated electromagnetic flowmeters is subject to variation as a function of temperature, fluid conductivity, pressure and other effects which can alter the spurious voltages both with respect to phase and magnitude. Hence, it becomes necessary periodically to manually re-zero the meter to correct for the effects on zero by the above-described phenomena.
All of the adverse effects encountered in a-c operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi.) /dt, serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent and zero drift effects would disappear. When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered.
The concern of the present invention is with a flowmeter of the type disclosed, for example, in German patent publication DT-AS 1,963,901, wherein a flow tube lined with dielectric material is provided with two or more planar electrodes which have a relatively large area, the electrodes being in electrically-conductive contact with the liquid passing through the tube. In flowmeters of this type, the usual practice is to employ a sinusoidal alternating field, as a consequence of which special compensation means are necessary to eliminate interference and, in particular, the interfering quadrature voltage.
These interfering quadrature voltages are applied to and act to overdrive the amplifying means connected in series with the measuring electrodes. Non-conductive coatings formed on the measuring electrodes and partially or fully covering these electrodes serve to change the valence field of the signal detection, especially in the case of large-area measuring electrodes. As a consequence, in addition to the interfering quadrature voltages, zero drift and other errors in the measuring voltage are encountered. To avoid these interfering effects, it is necessary to keep the surfaces of the measuring electrodes clean. For this purpose, use is made of cleaning devices which function electrically or mechanically.
Over and above the expenditures involved in employing these cleaning devices, a further disadvantage arises by reason of the fact that the mechanical elements of the cleaning devices act in combination with the measuring electrode leads to increase the stray capacitance of the signal lead out. As a secondary consequence of this arrangement, the useful range of the flowmeter is restricted with respect to the permissible lower limit of the electrical conductivity of the fluid being measured.
While such interfering quadrature voltages can be eliminated by exciting the flowmeter with square-shaped a-c fields, or by periodic d-c fields, as disclosed in U.S. Pat. Nos. 3,550,446; 3,329,018 and 3,786,687, or in German patent publication 2,052,175, asymmetric electrochemical voltages are generated in these arrangements as well as external direct voltages. These voltages must be rendered ineffective in order to prevent overdrive of the amplifying means coupled to the measuring electrodes.
In the presence of flowing liquid having abrasive properties, the measuring electrodes which are in direct conductive contact with these liquids must be fabricated of a material resistant to abrasion, for otherwise abrasion may in time result in the destruction of the electrodes. Moreover, corrosion and crystallization taking place in the slits of the insulation elements for the electrodes extending through the flow tube act to impair the effectiveness and life of the flow tube.
German Pat. No. 1,473,041 and U.S. Pat. No. 3,274,831 disclose a flowmeter energized by a sinusoidal high-frequency a-c field, the flowmeter having two or more planar electrodes of large area and a shield of even larger area cooperating with each electrode. This shielded-electrode flowmeter arrangement is usable to measure both non-conductive as well as conductive fluids, so that it may be employed to measure dielectric fluids.
For measuring dielectric fluids, the electrodes are covered by the dielectric lining of the flow tube and are thereby protected from the fluid. This lining, together with the dielectric of the fluid, constitutes the dielectric of a capacitor whose plates are formed by the planar measuring electrodes. In flowmeters of this type, stray capacitive effects may lead to erroneous measurements.